rt9218b Richtek Technology Corporation, rt9218b Datasheet - Page 13

• Manufacturer Part Number: rt9218b
• Description: Synchronous Buck Dc-dc Linear Power Controller
• Manufacturer: Richtek Technology Corporation

Under Voltage Protection
The voltage at FB and FBL pin is monitored and protected
against UV (under voltage). The UV threshold is the FB or
FBL
When OC or UV_FBL is trigged, a
will be initialized, as shown in Figure 7 Only 4 times of
trigger are allowed to latch off. Hiccup is disabled during
soft-start interval, but UV_FB has some difference from
OC and UV_FBL, it will always trigger V
after 4 times hiccup, as shown in Figure 8.
LDO Power Sequence
In VGA field, the MOSFET of LV
voltage not by SV
This connection may trigger UV protection to shutdown
RT9218B, but using the typical application circuit won't
have this issue. See figure 9 using OPS pin to control the
power sequence.
DS9218B-09 March 2007
UGATE
V
under 75%. UV detection has 30μs triggered delay.
0 A
4 V
2 V
0 V
OUT
V
FB
IN
Figure 8, UV_FB trigger V
Figure 7. UV and OC trigger hiccup mode
COUNT = 1
I
(20V/Div)
(500mV/Div)
OUT
T0
= 2A
T1
OUT
COUNT = 2
.
OVERLOAD
APPLIED
T2
Time (10ms/Div)
Power Off
TIME
COUNT = 3
OUT
IN
T3
hiccup restart sequence
power sensing
is sourced by external
UV
IN
power sensing
COUNT = 4
V
Sensing
IN
T4
(2V/Div)
(2V/Div)
Power
PWM Layout Considerations
MOSFETs switch very fast and efficiently. The speed with
which the current transitions from one device to another
causes voltage spikes across the interconnecting
impedances and parasitic circuit elements. The voltage
spikes can degrade efficiency and radiate noise, that results
in over-voltage stress on devices. Careful component
placement layout and printed circuit design can minimize
the voltage spikes induced in the converter. Consider, as
an example, the turn-off transition of the upper MOSFET
prior to turn-off, the upper MOSFET was carrying the full
load current. During turn-off, current stops flowing in the
upper MOSFET and is picked up by the low side MOSFET
or schottky diode. Any inductance in the switched current
path generates a large voltage spike during the switching
interval. Careful component selections, layout of the critical
components, and use shorter and wider PCB traces help
in minimizing the magnitude of voltage spikes.
There are two sets of critical components in a DC-DC
converter using the RT9218B. The switching power
components are most critical because they switch large
amounts of energy, and as such, they tend to generate
equally large amounts of noise. The critical small signal
components are those connected to sensitive nodes or
those supplying critical bypass current.
The power components and the PWM controller should
be placed firstly. Place the input capacitors, especially the
high-frequency ceramic decoupling capacitors, close to the
power switches. Place the output inductor and output
capacitors between the MOSFETs and the load. Also locate
the PWM controller near by MOSFETs.
A multi-layer printed circuit board is recommended.
Shutdown
Figure 9. LDO power sequence
Enable
RT9218B
www.richtek.com
VIN_SW (5V/12V)
VIN_LDO (3.3V)
OPS_Disable
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